1. Field of the Invention
The invention relates to a frictional engagement apparatus formed of a wet multiple disc clutch of a vehicular automatic transmission for example. More particularly, the invention relates to a frictional engagement apparatus that reduces drag torque generated when the clutch is not engaged.
2. Description of the Related Art
A wet multiple disc clutch typically includes a cylindrical clutch drum, a plurality of separator plates that are spline-engaged to the inner peripheral portion of the clutch drum, a clutch hub that is arranged radially inward of the clutch drum, and a plurality of friction plates that are spline-engaged to the outer peripheral portion of the clutch hub. The plurality of separator plates and the plurality of friction plates are alternately arranged in the axial direction. Lubricating oil is supplied to the plates so that they engage smoothly. The lubricating oil lubricates and cools the plates. However, when the plates are not engaged and one set of plates is rotating, shearing force is generated in the lubricating oil flowing between the separator plates and the friction plates. As a result, the non-rotating plates create drag on the rotating plates, creating rotational resistance. That is, drag torque is generated by the lubricating oil. This kind of drag torque reduces fuel efficiency, so attempts are being made to improve the structure to suppress this drag torque.
Japanese Patent Application Publication No. 10-169681 (JP-A-10-169681), for example, describes a frictional engagement apparatus that suppresses this kind of drag torque. The described apparatus includes a disc with internal teeth and a disc with external teeth, and friction material affixed to one side of either one of the discs in the axial direction. A groove that communicates the inner diameter portion with the outer diameter portion and which is inclined in the rotational direction of the disc with the internal teeth is formed in the friction material affixed to the disc with the external teeth, while a groove that is not inclined is formed in the friction material that is affixed to the disc with the internal teeth. In the frictional engagement apparatus, the disc with the internal teeth is engaged with the disc with the external teeth in such a manner that the side face of the friction material with the inclined groove faces the side face of the friction material with the non-inclined groove. With this structure, when the disc with the internal teeth and the disc with the external teeth are not engaged and the disc with the internal teeth is rotating faster than the disc with the external teeth, lubricating oil is forced out of both the inclined groove in the disc with the external teeth and the non-inclined groove in the disc with the internal teeth, thereby reducing the amount of lubricating oil on the friction surface of the disc with the external teeth, which in turn inhibits drag torque from increasing.
Also, Japanese Patent Application Publication No. 2001-234947 (JP-A-2001-234947), for example, describes a related frictional engagement apparatus that also suppresses this kind of drag torque. This apparatus includes a disc with external teeth that is spline-engaged to an outer member, and a disc with internal teeth that is spline-engaged to an inner member. The disc with the external teeth and the disc with the internal teeth are pressed against one another in the plate thickness direction such that the two discs are frictionally engaged. One of the discs, either the disc with the external teeth or the disc with the internal teeth, is formed by an undulating disc that is curved in a wave shape in the plate thickness direction, with the circumferential direction as the wavelength direction. At the initial stage of engagement in this frictional engagement apparatus, the mountain and valley portions of the waves elastically deform while the wave disc presses against the other disc. The buffering action from the elastic deformation of the mountain and valley portions prevents a rapid increase in torque, and also absorbs engagement shock. Also, during engagement and disengagement, the wave disc is quickly moved away from the other disc by the elastic restoring force of the mountain and valley portions, thereby suppressing drag torque from the lubricating oil.
However, with the frictional engagement apparatus described in JP-A-10-169681, power is transmitted by engaging the flat engagement surface of the friction material of the disc on which the internal teeth are formed with the flat engagement surface of the friction material of the disc on which the external teeth are formed. Therefore, the groove width is relatively small so the amount of lubricating oil that is discharged along this groove is small. As a result, if the disc with the internal teeth is not engaged with the disc with the external teeth, a relatively large amount of drag torque is generated by the lubricating oil in the gap between the two discs, and this drag torque is unable to be sufficiently suppressed. Also, if the disc with the internal teeth is not engaged with the disc with the external teeth and the relative rotational speed between those discs is high, lubricating oil is not easily discharged from the narrow groove in the friction material. As a result, neither the suppression of drag torque, not cooling from the lubricating oil, is promoted.
Also, with the frictional engagement apparatus described in JP-A-2001-234947, the wave-shaped concave and convex portions that are curved in the plate thickness direction with the circumferential direction as the wavelength direction are formed on either the disc with the external teeth or the disc with the internal teeth, and the ridge lines of the concave and convex portions are radially formed so as to pass through the center point of the disc. Therefore, when the disc with the internal teeth is not engaged with the disc with the external teeth and the relative rotational speed between those discs is low, drag torque is generated by lubricating oil in a gap between the disc with the internal teeth and the disc with the external teeth, which is defined by the concave and convex portions, even though that lubricating oil flows to the outer portion from that gap. Accordingly, drag torque cannot be sufficiently suppressed. Also, if the disc with the internal teeth is not engaged with the disc with the external teeth and the relative rotational speed between the discs is high, lubricating oil that flows through the gap between the disc with internal teeth and the disc with external teeth ends up colliding with the side wall surface because the ridge lines of the concave and convex portions are radially formed so that they pass through the center point of the disc with the external teeth or the disc with the internal teeth. This impedes the discharge of lubricating oil, so cooling is not promoted.